Block SSH Server Attacks (Brute Force Attacks) Using DenyHosts

DenyHosts is an open-source and free log-based intrusion prevention security program for SSH servers developed in Python language by Phil Schwartz.

It is intended to monitor and analyze SSH server logs for invalid login attempts, dictionary-based attacks, and brute force attacks by blocking the originating IP addresses by adding an entry to /etc/hosts.deny file on the server and prevents the IP address from making any further such login attempts.

[ You might also like: How to Secure and Harden OpenSSH Server ]

DenyHosts is much needed tool for all Linux based systems, specially when we are allowing password less ssh logins to remote Linux servers.

In this article, we are going to show you how to install and configure DenyHosts on RHEL-based Linux distributions such as Fedora, CentOS, Rocky Linux, and AlmaLinux.

See also :

How to Install DenyHosts in Linux

By default DenyHosts tool is not included in the Linux systems, we need to install it using third party EPEL repository. Once added repository, install the package using following yum command.

# yum install epel-release
# yum install denyhosts

Configuring DenyHosts for Whitelist IP Addresses

Once the Denyhosts installed, make sure to whitelist your own IP address, so you will never get locked out. To do this, open a file /etc/hosts.allow.

# vi /etc/hosts.allow

Below the description, add the each IP address one-by-one on a separate line, that you never want to block. The format should be as follows.

# hosts.allow   This file contains access rules which are used to
#               allow or deny connections to network services that
#               either use the tcp_wrappers library or that have been
#               started through a tcp_wrappers-enabled xinetd.
#               See 'man 5 hosts_options' and 'man 5 hosts_access'
#               for information on rule syntax.
#               See 'man tcpd' for information on tcp_wrappers

Configuring DenyHosts for Email Alerts

The main configuration file is located under /etc/denyhosts.conf. This file is used to send email alerts about suspicious logins and restricted hosts. Open this file using VI editor.

# vi /etc/denyhosts.conf

Search for the ‘ADMIN_EMAIL‘ and add your email address here to receive email alerts about suspicious logins (for multiple email alerts use comma separated). Please have a look at the configuration file of my CentOS 6.3 server. Each variable is well documented so configure it according to your liking.

############ DENYHOSTS REQUIRED SETTINGS ############
SECURE_LOG = /var/log/secure
HOSTS_DENY = /etc/hosts.deny
WORK_DIR = /var/lib/denyhosts
LOCK_FILE = /var/lock/subsys/denyhosts

############ DENYHOSTS OPTIONAL SETTINGS ############
ADMIN_EMAIL = [email protected]
SMTP_HOST = localhost
SMTP_FROM = DenyHosts <[email protected]>
SMTP_SUBJECT = DenyHosts Daily Report

############ DENYHOSTS OPTIONAL SETTINGS ############
DAEMON_LOG = /var/log/denyhosts

Restarting DenyHosts Service

Once you’ve done with your configuration, restart the denyhosts service for new changes. We also add the denyhosts service to system start-up.

# chkconfig denyhosts on
# service denyhosts start

Watch DenyHosts Logs

To watch denyhosts ssh logs for how many attackers and hackers are attempted to gain access to your server. Use the following command to view the real-time logs.

# tail -f /var/log/secure
Nov 28 15:01:43 tecmint sshd[25474]: Accepted password for root from port 4339 ssh2
Nov 28 15:01:43 tecmint sshd[25474]: pam_unix(sshd:session): session opened for user root by (uid=0)
Nov 28 16:44:09 tecmint sshd[25474]: pam_unix(sshd:session): session closed for user root
Nov 29 11:08:56 tecmint sshd[31669]: Accepted password for root from port 2957 ssh2
Nov 29 11:08:56 tecmint sshd[31669]: pam_unix(sshd:session): session opened for user root by (uid=0)
Nov 29 11:12:00 tecmint atd[3417]: pam_unix(atd:session): session opened for user root by (uid=0)
Nov 29 11:12:00 tecmint atd[3417]: pam_unix(atd:session): session closed for user root
Nov 29 11:26:42 tecmint sshd[31669]: pam_unix(sshd:session): session closed for user root
Nov 29 12:54:17 tecmint sshd[7480]: Accepted password for root from port 1787 ssh2

Remove Banned IP Address from DenyHosts

If you’ve ever blocked accidentally and want to remove that banned IP address from the denyhosts. You need to stop the service.

# /etc/init.d/denyhosts stop

To remove or delete banned IP address completely. You need to edit the following files and remove the IP address.

# vi /etc/hosts.deny
# vi /var/lib/denyhosts/hosts
# vi /var/lib/denyhosts/hosts-restricted
# vi /var/lib/denyhosts/hosts-root
# vi /var/lib/denyhosts/hosts-valid
# vi /var/lib/denyhosts/users-hosts

After removing the banned IP Address, restart the service again.

# /etc/init.d/denyhosts start

The offending IP address added to all the files under /var/lib/denyhosts directory, so it’s makes very difficult to determine the which files contain the offending IP address. One of the best way to find out the IP address using grep command. For example to find out IP address, do.

cd /var/lib/denyhosts
grep *

Whitelist IP Addresses Permanently in DenyHosts

If you’ve list of static IP address that you want to whitelist permanently. Open the file /var/lib/denyhosts/allowed-hosts file. Whatever IP address included in this file will not be banned by default (consider this as a whilelist).

# vi /var/lib/denyhosts/allowed-hosts

And add the each IP address on separate line. Save and close the file.

# We mustn't block localhost
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18 thoughts on “Block SSH Server Attacks (Brute Force Attacks) Using DenyHosts”

  1. # cat /etc/redhat-release
    CentOS Linux release 8.5.2111

    # rpm -qa | grep epel-release

    # yum install denyhosts
    No match for argument: denyhosts

  2. This is good. I took it one step further too. I took all the blocklists maintained by and saved them into a single file, then wrote this shell script to filter the comments out of the list and use iptables to block entire ranges:

    while read line; do
        echo " "
        echo "Current Line: $line"
        if [[ ${line:0:1} == [0-9]* ]]; then        
            iptables -I INPUT -s $line -j DROP
            echo "$line is a valid IP range. Added to iptables block list."
            echo "$line was skipped. Not a valid IP."
    done <"$file"
  3. Thanks for the info! What happens if you log in from an ISP that changes your IP once in a while? You’d be locked out of the whitelist unbeknown to you. Any way around this apart from console access?


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